1. Field of the Invention
The present invention relates generally to petroleum wells and in particular to petroleum wells having a communication system for delivering power and communications to a downhole hydraulic system, the hydraulic system being operably connected to a downhole device for operating the downhole device.
2. Description of Related Art
Several methods have been devised to place electronics, sensors, or controllable valve downhole along an oil production tubing string, but all such known devices typically use a internal or external cable along the tubing string to provide power and communications downhole. It is, of course, highly undesirable and in practice difficult to use a cable along the tubing string either integral to the tubing string or spaced in the annulus between the tubing string and the casing. The use of a cable presents difficulties for well operators while assembling and inserting the tubing string into a borehole. Additionally, the cable is subjected to corrosion and heavy wear due to movement of the tubing string within the borehole. An example of a downhold communication system using a cable is shown in PCT/EP97/01621.
U.S. Pat. No. 4,839,644 describes a method and system for wireless two-way communications in a cased borehole having a tubing string. However, this system describes communication scheme for coupling electromagnetic energy in a TEM mode using the annulus between the casing and the tubing. This inductive coupling requires a substantially nonconductive fluid such as crude oil in the annulus between the casing and the tubing. Therefore, the invention described in U.S. Pat. No. 4,839,644 has not been widely adopted as a practical scheme for downhole two-way communication. Another system for downhole communication using mud pulse telemetry is described in U.S. Pat. Nos. 4,648,471 and 5,887,657. Although mud pulse telemetry can be successful at low data rates, it is of limited usefulness where high data rates are required or where it is undesirable to have complex, mud pulse telemetry equipment downhole. Other methods of communicating within a borehole are described in U.S. Pat. Nos. 4,468,665; 4,578,675; 4,739,325; 5,130,706; 5,467,083; 5,493,288; 5,576,703; 5,574,374; and 5,883,516. Similarly, several permanent downhole sensors and control systems have been described in U.S. Pat. Nos. 4,972,704; 5,001,675; 5,134,285; 5,278,758; 5,662,165; 5,730,219; 5,934,371; and 5,941,307.
The Related Applications describe methods for providing electrical power and communications to various downhole devices in petroleum wells. These methods use either the production tubing as a supply and the casing as a return for the power and communications transmission circuit, or alternatively, the casing as the supply with a formation ground as the return. In either configuration, electrical losses in the transmission circuit are highly variable, depending on the specific conditions for a particular well. Power supplied along the casing with a formation ground as the return is especially susceptible to current losses. Electric current leakage generally occurs through the completion cement into the earthen formation. The more conductive the cement and earthen formation, the greater the current loss as the current travels along the casing.
A need therefore exists to accommodate power losses which will be experienced when using a downhole wireless communication system. Since these losses place limits on the available amount of instantaneous electrical power, a need also exists for a system and method of storing energy for later use with downhole devices, especially high energy devices such as emergency shutoff valves, or other safety equipment. Although one solution to downhole energy storage problems could be provided by electrical storage such as capacitors, or chemical storage such as batteries, the limited lifetimes of such devices makes the use of the devices less than ideal in an operating petroleum well.
All references cited herein are incorporated by reference to the maximum extent allowable by law. To the extent a reference may not be filly incorporated herein, it is incorporated by reference for background purposes and indicative of the knowledge of one of ordinary skill in the art.